Bug 26043: Fix random failure from Holds.t

Bug 26043: Fix random failure from Holds.t

 #   Failed test 'Test ModReserveMinusPriority()'
 #   at t/db_dependent/Holds.t line 202.
 #          got: undef
 #     expected: '1605'
 # Looks like you failed 1 test of 66.

It is coming from Koha::Patron->holds that is ordering by reservedate,
so "sometimes" they are ordered in reverse (at least it's my
understanding of the problem).

Test plan:
Run the test file several times (from 20 to 60x), it must never fail
with this patch

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>